Death's Shadow gets -X/-X, where X is your life total.
Death's Shadow's ability applies only while Death's Shadow is on the battlefield. In all other zones, its power and toughness are 13.
If your life total is negative, X is considered to be 0.
In a Two-Headed Giant game, your life total is your team's life total.
The value of X changes as you gain and lose life. It's not locked in as Death's Shadow enters the battlefield.